In my last article I ended with an explanation of the assumptions which are made in the standard rating of slings. I tried to indicate the situations where an allowance is needed without attempting to quantify how much that should be. This month I look further at rating of slings and loose lifting gear and add some detail so as to help quantify the effects of deviating from the rating assumptions.

More about rating
In summary, all slings are rated for their legs in straight pull. The standard ratings for multi-leg slings assume that all sling legs are in use, that they are all at the same angle to the vertical and that angle is within the rated range and that when viewed from above, all the legs are equally disposed. Finally in the case of four leg slings, the load has a degree of flexibility so as to share the load between the four legs.

In practice it is unlikely that such conditions are met exactly but then it is also unlikely that the sling will be loaded to its maximum capacity. There is a degree of tolerance and, as a general guide, the loading on a multi-leg sling can be regarded as within the rating assumptions if all the following conditions are met:

(a) all sling legs are in use;
(b) the load is less than 80% of the marked SWL;
(c) none of the sling legs exceed the maximum permitted angle to the vertical for the marked SWL;
(d) the sling leg angles to the vertical are all at least 15°;
(e) the sling leg angles to the vertical are all within 15° to each other;
(f) in the case of three leg slings, when viewed from above the angles between adjacent legs are all within 15° of each other;
(g) in the case of four leg slings, whenviewed from above the angles between any pair of adjacent legs are within 15° of the opposite pair.

The most likely deviations from these conditions are that not all legs are in use or that one or more legs is in choke hitch. If not all legs are in use, the simplest allowance to make is to reduce the safe working load in proportion to the number of legs in use. For example if only two legs of a four leg sling are in use then reduce the SWL by half. This does mean that in some applications the sling will be under utilised but it is an easy rule to remember and apply.

When a sling leg is wrapped around the load and hooked back on itself or reeved through itself, that is what we call choke hitch. As the tension is taken up the choke tightens naturally until the three parts of the leg which meet at the point of choke are at 120º to each other. Local stresses at the point of choke make it necessary to derate the sling. The effect varies with the type of sling but, for simplicity, the industry has standardised on a de-rating factor of 0.8 for all sling types. So, whenever a sling has a leg to be used in choke hitch, multiply the standard rating by 0.8 to get the SWL you can use.

Although this article is intended as a buyer’s guide, a word of caution about the way choke hitches are applied is appropriate here. They are often used to connect to bundles of loose items such as tubes or reinforcing bar. When left to take up the natural choke angle, the items in the triangular area just below the choke are not gripped and can fall out if the load tips. In order to secure them, slingers often use an implement to ‘batter down’ the choke.

This is a very dangerous practice. It never will secure all the items but more importantly, it increases the angle below the choke to considerably more than the natural 120º. The effect is the same as using a two leg chain sling at such an excessive angle. The sling is locally overloaded and might fail. The correct way to secure all the bundle, and this is what makes it relevant to the buyer, is to use a sling with a longer leg and wrap it a complete turn around the bundle before choking it. The de-rating factor is still 0.8 but the ‘wrap and choke’ method secures the bundle far better without the detrimental effect of battering down.

Multi-leg slings are marked with a rating for angular use but single leg slings are not. Nevertheless they are often used in combination and that is acceptable practice provided the appropriate allowances are made. In applications where the maximum utilisation is necessary and the precise angles of use can be predicted, then a little trigonometry will enable the required capacity of slings to be established exactly. However for general purposes, the same factors used for general purpose multi-leg slings can be used and the results will be applicable for a range of angles 0-45º to the vertical.

The factors for a range of angles 0-45º to the vertical are 1.4 for two leg slings and 2.1 for three and four leg slings. So, starting with the weight of the load, dividing by the appropriate factor gives the minimum SWL required for each of the slings if used in straight pull. If the connection is to be by choke hitch, divide that answer by 0.8 to get the final SWL required.

Single leg slings and round slings can also be used to advantage in combination with multi-leg slings. For example to lift a machined shaft, a two leg chain sling can be hooked into two round slings, one each end of the shaft in choke hitch to provide attachment points. The chain sling with shortening clutches provides ease of adjustment and the roundslings provide the means of attachment without damaging the machined surface.

When using two or more slings in combination, it is good practice to connect them to the crane hook with a shackle. This prevents overcrowding of the hook and also ensures that the load is transmitted to the hook through its seat. Connecting several slings, each loaded at an angle, direct to the hook loads the hook in a way that tries to open it.

Shackles
The shackle is a simple but very versatile lifting accessory with many possible applications. A good selection of shackles is an essential part of every rigging store. There are numerous national and international standards for shackles around the world. In the UK we have BS 3032 for higher tensile steel shackles and BS EN 13889. Although BS 3032 is now withdrawn in favour of the harmonized European Standard BS EN 13889, shackles are still manufactured to that standard.

Shackles are graded according to the mean stress at the minimum breaking load in the same way as chain and chain slings. Essentially the higher the grade, the greater the capacity for a given size. Shackles to BS 3032 are grade 4 whereas those to EN 13889 are grade 6. Grade 8 shackles are available but are less popular. The reason is that in many applications, it is the dimensions of the shackle which are the limiting factor rather than the SWL. For example if connecting two single leg wire rope slings to a crane hook, the shackle must be big enough to go on the crane hook and accommodate the two slings.

All shackles comprise a body and a pin. There are two main variations of body and two of pin. The basic body shapes are ‘D’ (or sometimes dee) and bow. A D shackle has long straight parallel sides and a semicircular end, the pin representing the upright part of the letter D. The end of a bow shackle is wider forming a larger segment of a circular ring, narrowing to short straight sides leading to the pin.

The bow shackle is generally the most versatile as it can be used in straight pull but the bow end is large enough to accommodate two or possibly more mating slings and components. The D shackle is only for use in straight pull but is rather more compact. It is best suited to applications such as lifting beams and lower terminal fittings for wire rope slings. In older standards such as BS 3032, all the dimensions are fully specified. Moreover both types had small and large versions, the essential difference being in the jaw width. The narrow jaw versions were primarily for use with specific mating components such as sheave blocks whereas the wider jaw versions were for general purpose applications. Modern standards such as BS EN 13889 are performance based and only specify as maxima and minima those dimensions critical for compatibility.

The two variations of pin are ‘screwed’ and ‘nut and bolt’. A screwed pin has a head at one end and a thread at the other. One eye of the shackle body has a plain hole and the other threaded. This keeps the number of components to the minimum of two and it is quick and relatively easy to dismantle and assemble. However under some circumstances, this type of pin can have a tendency to unscrew. In applications where the shackle is left in place for a long time and the load is repeatedly applied and released, the mating component can grip the pin and turn it a little with each application of load. It is a ratchet effect which gradually unscrews the pin.

There was a fatal accident a few years ago due to this effect. A two leg sling was connected by shackles to pipe hooks. A cargo of pipes was unloaded in the course of which the ratchet effect tightened the pin of one shackle but slackened the other with tragic consequences.

For longer term applications, particularly where the shackle cannot easily be seen, the nut and bolt type of pin is more suitable. With this type, both eyes of the shackle body have plain holes. The bolt passes through both and the nut is secured with a split cotter pin. When correctly assembled the pin is free to rotate within the shackle body but remains secure. Shackle bodies and pins are supplied as a matched pair and clearly, because of the difference in the body holes, the pin types are not interchangeable.

When purchasing shackles, the buyer must consider not only the SWL required, but also the dimensions necessary for compatibility with mating components. The nature of the application will determine the type of pin required.